Known in the art are gold-plating electrolytes based on sodium or potassium dicyanoaurate and containing also potassium sulphate, potassium hydrogen phosphate, an alkali metal salt of ethylenediaminoacetic acid which are prepared by anodic or chemical dissolution of gold in solutions of alkali metal cyanides.
Preparation and use of this type electrolytes are associated with evolution of toxic compounds and the necessity of making them harmless. Said prior art electrolytes make it possible to obtain a good-quality coating only at large thicknesses thereof.
Preparation and use of said prior art gold-plating electrolytes is also accompanied by difficulties encountered in recovery of gold from spent and mother liquors and outgassing of toxic compounds.
Most widely employed gold-plating electrolytes are based on disulphitoaurate, ethylenediaminodisulphitoaurate and diaminodisulphitoaurate of alkali metals; properties of these gold-plating electrolytes depend on the method of preparing same. This method comprises conversion of aurichlorohydric acid to a sparingly soluble gold compound, auric hydroxide or auric ammoniate to purify from Cl.sup.- ions; this gold compound is treated with solutions of sulphites of alkali metals, followed by stabilization of the resulting sulphite complexes in the solution.
The thus-produced sparingly soluble gold compounds are low-stable thus causing a partial evolution of metal gold which, in turn, lowers the desired product yield. This also hinders preparation of highly-concentrated electrolytes for gold-plating which would be stable in storage and use, whereby the efficiency of the gold-plating process is educed.
Known in the art is a gold-plating electrolyte (cf. U.S. Pat. No.3,893,896) based on potassium disulphitaurate and having the following composition:
K.sub.3 [Au(SO.sub.3).sub.2] : 6.8-68 PA1 Na.sub.2 H.sub.2 Edta: 0.1-100 PA1 K.sub.2 SO.sub.4 : 1.0-100 PA1 K.sub.2 HPO.sub.4 : 10-40 PA1 water: the balance PA1 hexapotassium .mu.-ethylenediaminetetracetate bis(gold sulphite /l/): 10.5-123.0 PA1 an alkali metal salt of ethylenediaminetetracetic acid: 17.8-140.0 PA1 potassium sulphate: 12.6-110.0 PA1 potassium hydrogen phosphate: 4.7-40.0 PA1 water: the balance PA1 hexapotassium .mu.-ethylenediaminetetracetate bis(aurous sulphite): 10.5-25.0 PA1 disodium-dipotassium salt of ethylenediaminetetracetic acid: 17.8-40.0 PA1 potassium sulphate: 12.6-28.4 PA1 potassium hydrogen phosphate: 4.7-10.4 PA1 water: the balance PA1 K.sub.6 [Au(SO.sub.3).sub.2 Edta]: 10.5-123 PA1 K.sub.2 Na.sub.2 Edta: 17.8-140 PA1 K.sub.2 SO.sub.4 : 12.6-110 PA1 K.sub.2 HPO.sub.4 : 4.7-40 PA1 water: the balance
(current density is D.sub.k =0.1-1.2 A/dm.sup.2 ; T=18.degree.-80.degree. C.).
This gold-plating electrolyte has a throwing power of up to 90%, yield current up to 70-80%, period of storage at the temperature of 25.degree.0 C. as long as 4 months, starting point of decomposition at D.sub.k above 0.8 A/dm.sup.2.
This electrolyte ensures the manufacture of high-quality fine coatings with Vickers hardness of from 80 to 220 kg/mm.sup.2, porosity of from 25 to 30 pores/cm.sup.2 at the thickness of 15 mcm and non-porous at a thickness of below 5 mcm.
The process for the preparation of this electrolyte comprises treatment of aurichlorohydric acid with a solution of ammonia at a pH of from 3 to 6 to give basic auric ammoniate, followed by decantation of the mother liquor and washing in hot water. The resulting suspension is added to a hot solution of potassium sulphite (to the concentration of 150 g/l), followed by heat-treatment of the resulting mixture at a temperature ranging from 70.degree. to 90.degree. C. for a period of from 48 to 72 hours and addition of an alkali metal salt of ethylenediaminetetracetic acid, potassium sulphate and potassium hydrogen phosphate. The resulting eletrolyte is filtered-off to recover partly precipitated metallic gold. The yield of the thus-produced gold-plating electrolyte is 30 to 60% as calculated for metal gold.
This prior art gold-plating electrolyte has a reduced stability in storage and use, as well as insufficient technological parameters.